Set up a private KOTH match on Checkout. Score to 240, rounds to 1, no bots, change all weapon spawns to the Longshot Sniper Rifle, friendly fire doesn't matter and weapon spawns to after pick up.

Now with the signed in player, or friend, go to where the ink usually is, pick up sniper and look at the opposite spawn you started to get the first kill. Now just look at where the digger/sniper spawn is and your main player will keep spawning there. Just make sure that the person sniping on the ledge where ink spawn is that they are back to the wall, if they go forward a bit it will mess up the spawns!

Just keep headshotting and every 5 kills the main player going for the medal just runs up and executes them. Then picks up there sniper. The other player then can run and kill you and pick up the dropped sniper again and start the process over.

The match will end automatically after 20 minutes but you can get around 10 killed nemesis each match. You need 100 for Onyx.